Post-1980 Proxies #3: Chesapeake Bay Mg/Ca

The Chesapeake Bay Mg/Ca proxy goes to late 1995 and is used in both Moberg et al [2005] and Mann and Jones [2003] for the proxy reconstructions up to 1980. Figure 1 below shows that its post-1980 behavior does not show an exceptional response to supposedly unprecedented temperature.
Figure 1. Chesapeake Bay Spring SST. Re-plotted from data at WDCP . See Figure 4 in article.

The Mg/Ca proxy for spring sea surface temperature "relies on the thermodependence of the uptake of magnesium into the calcitic shells of microfossils (Dwyer et al., 1995; Lear et al., 2000). Mg/Ca ratios in ostracode (Crustacea) shells are influenced by the water temperature in which the shell is secreted and the Mg/Ca ratio in the water." [Cronin et al, 2003]. The data goes up to late 1995 and does not include the warm 1998. However, it is evident that there is no unusual response of this proxy. The plot of the 19th and 20th century data looks more volatile, but Cronin et al. [2003] do not discuss whether this is an artifact of sampling. One sees increases in volatility in the modern portion of ice cores, where the change is attributed to isotope migration. One wonders whether a similar phenomenon may be at work here.

Both Moberg et al. [2005] and Mann and Jones [2003] splice CRU surface temperature histories after 1980 to their proxy reconstructions and the post-1980 proxy information is not used in these studies. Jones and Mann [2004] does discuss proxy behavior after 1980, claiming that the proxies from Mann and Jones [2003] also show unprecedented warming after 1980. A simple inspection of this record shows that it is obviously not driving the Jones and Mann [2004] claims. I think that I’ll show a couple more proxies and will make some comments on Mann and Jones [2003] and Jones and Mann [2004] in a few days.

Cronin et al. contains the following interesting short history of temperature in the Chesapeake Bay, comparing local measurements and COADs measurements, with the COADs measurements diverging on the warm side in the later 1990s. I’m not familiar enough with the temperature datasets to comment on this issue, but found it interesting.

7 Comments

One thing that Real Climate mentions about this paper is that the authors consider tree ring data to be useful for short-term fluctuations, but not for long-term trends greater than 80 years. The paper evidently combines longer-term data from sediment cores with shorter-term tree ring data in the graph.

My response to realclimate is: useful for what? Trees, we are told, are not sensitive to local temperature conditions, yet by some strange magic, can be used to measure something called the “global temperature”.

What is the implication of figure 2b (can’t really read it). Are there two series on here that are moving together? If so, can’t some numeric analysis (covariation?) be made?

Also, where is the feild study proof of the proxy relevance (direct measurement of temp and Mg/Ca) or at least the covariance versus local temp stations in 1900s? Does Steve agree that this proxy “works” in these terms?

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[…] discussion of the topic from the early days of CA. Fourth series to enter the MJ03 portfolio is the Chesapeake Bay Mg/Ca proxy, also present in Figure 2. This series has declining temperatures since late 19th century. Finally, […]